A conventional gas turbine engine includes a compressor for providing compressed air to a combustor wherein it is mixed with fuel and ignited for generating combustion gases. The hot combustion gases are channeled to turbine blade-rows for extracting energy therefrom for powering the compressor and for generating thrust in propelling an aircraft, for example. All of the turbine flowpaths which channel the combustion gases are heated thereby and are subject to thermally induced stress therein. Since a turbine is axisymmetrical, the many annular structures therein are subject to radial thermal growth which increases their diameter and circumference when heated by the combustion gases. In order to reduce the thermal growth and stresses therein, the flowpaths are typically configured as circumferentially segmented components which permit unrestrained circumferential, or tangential, thermal expansion of the individual components, as well as radial thermal expansion thereof.
For example, conventional stator shrouds surrounding turbine blade-rows comprise a plurality of circumferentially adjoining individual shrouds or shroud segments which are conventionally joined to an annular outer casing. The inner surfaces of the shrouds form the radially outer flowpath of the combustion gases, and the outer surfaces of the shrouds are provided with cooling air channeled through the outer casing for cooling the shrouds. Since the outer casing is an annular structure maintained at a lower temperature than the shrouds being heated by the combustion gases, it expands radially outwardly at a different rate than that of the shrouds. If the shrouds were a fully annular, non-segmented structure, considerable thermal stress would be generated therein due, in part, to the restraint provided by the outer casing to which the shroud is attached.
By circumferentially segmenting the shroud, it is allowed to freely expand and contract in the circumferential direction which reduces thermal stress therein. However, suitable seals must then be provided between the individual segmented shrouds to prevent escape of the combustion gases radially outwardly toward the outer casing as well as prevent leakage of the compressed cooling air radially inwardly therebetween into the hot combustion gases. Sealing between the segmented shrouds is typically accomplished with conventional spline seals which are straight members disposed in complementary, U-shaped circumferentially facing slots disposed in the shroud edges. A predetermined radial gap is provided between the spline seal and its complementary slots for allowing alignment and assembly of the adjacent shrouds while still providing effective sealing therebetween.
A conventional turbine nozzle provides another example of a segmented flowpath wherein the individual nozzle stator vanes include radially outer and inner bands formed integrally with the vanes, which bands have a large circumferential overhang relative to the airfoil to adjoin adjacent vanes. The circumferential edges of the vane bands are also sealed using conventional spline seals.
As the diameter of the turbine flowpaths increase for larger and larger gas turbine engines, several significant problems arise. For example, relatively large circumferential and axial dimensions of individual flowpath segments create unavoidable fit-up difficulties at the segment edges during assembly. Both manufacturing variations and operation induced creep deformations of the segments increase the difficulty of alignment and installation of the axially extending spline seals therebetween. In the example of the nozzle vanes, large circumferential overhang of the vane bands increases the potential for dimensional variations thereof resulting in radial misalignment between adjacent vane bands.
The misalignment of adjacent bands significantly reduces the ability of the spline seals to form an effective seal therebetween. Conventional spline seals require two flat and parallel surfaces for the backside pressure provided by the compressed air to load the spline seal itself radially inwardly in its respective slot to form an effective seal therewith. Misalignment of the slot increases leakage past the spline seal significantly.
The relatively large length of the axial splitline in large diameter applications, increases the difficulty of spline seal design especially where the required joint to be sealed is nonlinear. Since spline seals must be straight in the axial plane of the engine in order to effectively seal a large number of axially spaced apart spline seals must be used to follow an axially varying or arcuate flowpath contour to provide effective sealing between the circumferentially adjacent components. A highly curved axial flowpath also increases the difficulty of radial alignment of the adjacent components during initial assembly. And, during operation as the components vary in configuration due to creep deformations thereof, maintaining alignment between the components becomes more difficult, and the ability of the spline seals to effectively seal the flowpath is further increased.
Tests have shown that spline seals in conventional shroud and nozzle applications, have relatively large leakage rates consistent with an effective 0.05 mm gap at the sealing surface. As the axial length of the sealed joint increases, leakage rates are further increased.